Printers use a halftone pattern to express brightness when they print an image. In other words, printers express an image by printing out a plurality of dots on paper. In this regard, expressing brightness and medium density of the image according to how densely the dots are printed on a certain portion by adjusting the number of dots is referred to as halftoning, and a pattern formed as such is referred to as a halftone pattern.
In a case where an original document printed by a printer and including a halftone pattern is copied, if a scan image is obtained by optically scanning the original document, and halftoning is performed on the obtained scan image according to a resolution of a photocopier, a final output document may have a moiré phenomenon due to halftoning frequency interference between the printer, which has printed the original document, and the photocopier.
Accordingly, in order to prevent such a moiré phenomenon occurring, work for removing a halftone pattern included in a scan image, that is, descreening work, is performed before halftoning is performed on the scan image according to a resolution of a photocopier. However, when descreening work is uniformly performed without taking frequency characteristics of the halftone pattern into account, the halftone pattern may not be properly removed, and thus, the moiré phenomenon may occur.